Managing tomorrow’s disease using yesterday’s methods

The time for improving vector control to prevent dengue outbreaks is now. “Dengue is the disease of the future,” comments Malaysian Public Health Medical Entomologist Dr. Moh-Seng Chang. “And it’s going to become more and more of a global health problem as international travel, urbanization, and transportation increase.”

With more than 40 years working in dengue vector control (which is one of the methods of dengue prevention), Dr. Chang has seen little advancement. He firmly believes that better evaluation, monitoring, and management are the keys to improving the outcome of vector control interventions. We spoke with him about vector control and his experiences in the last 40 years.

Dengue vector control in the 1970s

The year 1975 saw the US Watergate scandal, the manned American Apollo spacecraft, and the manned Soviet Soyuz spacecraft orbiting, and the last naturally occurring case of smallpox.It was also the year after a major dengue epidemic in Malaysia and other Southeast Asian nations; and the year when Dr. Chang started work as a medical entomologist at the Ministry of Health in Sarawak, Malaysia. Back in 1975, Dr. Chang had studied the mosquito vectors but, like many people knew very little about vector control operations.

Then, like now, there were two main sides to dengue vector control:

Breeding site reduction Governments were using radio, TV, newspapers, and leaflets to broadcast the call to clean up and to manage water storage. But their methods were falling on deaf ears. Unengaged, communities failed to act.

Chemical control:mounted on trucks or using fogging and larvicide Insecticide sprays weren’t reaching the inside of homes where mosquito vectors were latent and people, especially children, were found resting during the day. Vector mosquitoes simply built up resistance to common insecticides use – and continue to do so today. And the use of insecticides for larval control simply failed because there were too many cryptic breeding sites around and many of them couldn’t even be found during house inspections.

These vector control methods were not sustainable. One of the key problems, then and now, was that they were not properly evaluated or monitored. “The trucks continued spraying areas even after mosquitoes had built up resistance or were spraying in the wrong areas at the wrong times,” recalls Dr. Chang. “To be sustainable and effective, dengue vector control programs should be community-based. Dengue control programs must first evaluate each method’s suitability and acceptability, and then monitor its effectiveness over time. Dengue control needs to be preemptive – it needs to be implemented before outbreaks actually happen.”

Evaluation of larval and adult mosquito densities during the post-spraying operation was almost non-existent at that time – and still today. It’s not surprising that teams fighting dengue fever continue to spray insecticide without any thought as to whether they were actually having an impact.

Cheap, simple and accepted

Over the years Dr. Chang has come across some often quite simple vector control interventions that have worked well and recommended them for further field evaluation on a larger scale:

Encouraging farmers to cut the ends off the cocoa pods they split in half during harvesting not only stops the pods from filling with water; it makes it easier for the farmers to extract the cocoa seed.

Adding two guppy fish to water storage tanks or cement jars ensures any mosquito larvae are eliminated – eaten by the predacious fish – before they have the chance to mature. The guppies live for more than a year.

Image courtesy of Dr. Chang

In certain rural areas, where water supply is a problem and communities have to depend on rainwater from the roof, selling water tanks with a mosquito-proof cover stops the mosquito breeding cycle in its tracks. If a mosquito lays any eggs on the cover or in the roof gutters, they cannot fall into the water tank, while any mosquitoes that do mature within the tank cannot escape through the net.

These vector control methods worked because they were cheap, simple, and well-accepted by the community: they were sustainable. Another method, which didn’t work quite as well, was a program providing covers for smaller round water jars.

The netting covers were expensive and didn’t last long enough – less than a year in fact – making them too costly. “If the community could make the covers themselves or if the private sector became involved and came up with a cover that could be sold, with the jars at an affordable price, it would make this method much better,” adds Dr. Chang.

Image courtesy of Dr. Chang

Dr. Chang has some clear views on how things could work better. Check out our next article, to find out more…